1. Field of the Invention
The present invention relates to a process for the isolation of purified ethylene glycol carbonate (EGC) from a contaminated EGC which contains impurities from the group comprising starting materials, by-products and/or catalysts of the preparation process, in which the contaminated EGC is subjected to a fractional melt crystallization. The crystals of purified EGC formed in this case are then mechanically separated off from the remaining impurities dissolved in the residual melt. This novel isolation method is energy-saving and material-saving and does not require use of additives, for example solvents. As a result, particularly high degrees of purity of the EGC and the virtually complete reuse of the preparation catalyst are made possible.
2. Description of the Related Art
It is known that the EGC can be prepared by various routes, for example from ethylene glycol and phosgene (DE-AS (German Published Specification) 12 26 117), from ethylene oxide or ethylene chlorohydrin and carbon dioxide (Chem. Ing. Techn. 43 (1971), 903 ff.; Fette, Seifen, Anstrichmittel 73 (1971), 396 fl.; DE-OS (German Published Specification) 28 55 232, equivalent to U.S. Pat. No. 4,314,945; Ind. Eng. Chem. 50 (1958), 767-77.0) and from ethylene, oxygen and carbon dioxide (U.S. Pat. No. 3,025,305). In these cases, the EGC generally arises in a form contaminated by the various substances from the preparation process. In the process starting from ethylene oxide and carbon dioxide, the crude product further contains the dissolved catalyst, for example quarternary ammonium compounds.
A number of purification processes for the EGC have therefore become known. Such a purification is carried out in the majority of cases by vacuum distillation. In this vacuum distillation, in the case of contamination of the EGC with the starting material or by-product glycol, an azeotropic mixture of glycol and EGC passes over the column head, while residual EGC is obtained at the column foot. If the crude EGC to be purified additionally contains a dissolved catalyst from the preparation process, this must be separated off before the vacuum distillation described for the removal of glycol from the crude product. Thus, for example, in the process described in Chem. Ing. Techn. and Fette, Seifen, Anstrichmittel (loc. cit.) two thin-film distillations are first carried out to separate off the catalyst. Most of this catalyst separated off can then be returned back to the preparation reaction to give EGC. In the thin-film evaporators, small amounts of by-products are simultaneously separated off. Only after these thin-film distillations is the above-described azeotropic distillation for the removal of glycol carried out and the EGC obtained at the column foot is fed to a further rectification. By means of this procedure, starting from a crude product containing 96-98% EGC according to gas-chromatographic analysis, a pure product is achieved containing 99.5% EGC, and further containing 0.025% water and 0.1% glycol (again according to gas-chromatographic analysis).
Distillation processes for the purification of EGC generally have the disadvantage that a vacuum distillation system must be operated at pressures of at most 50 mbar, since, at higher pressures, which simultaneously correspond to higher distillation temperatures, the EGC is already partially decomposed and thus a reduction of the yield must be expected. This risk of the decomposition of the EGC is present in particular when the crude product still contains, in dissolved form, the catalyst from the reaction, for example of ethylene oxide with carbon dioxide, and this catalyst is likewise separated off by distillation before the fine purification by distillation. Thus, for example, in Ind. Eng. Chem. (loc. cit.) it is described that the pressure mentioned of at most 50 mbar is advisable in order to ensure a good product quality. However, since the catalyst is concentrated in the distillation bottoms by distilling off the pure EGC and, on the other hand, the degree of decomposition of the EGC depends strongly on the concentration of the catalyst dissolved therein, according to the said publication, at an input concentration of 0.25 to 0.5% by weight of catalyst, no more than 90 to 95% of the EGC must be distilled off. A further risk is that the catalyst itself can be decomposed during this distillation operation. If it is thus desired to return the distillation bottoms, which represent a concentrated catalyst solution, to the preparation reaction, a portion of the catalyst must be discarded and replaced by new catalyst. In the said publication, for example, replacement of 30% of the catalyst solution is recommended.
It is further known to purify EGC by extraction with solvents. In this case, a solvent mixture, a single solvent, such as ethylene dichloride, methylene dichloride or chloroform, or a hydrocarbon together with water (to receive the water-soluble catalyst) can be used (U.S. Pat. No. 2,766,258). In this case a degree of purity of 99.1% EGC is achieved. The disadvantage of an extraction process is the necessary subsequent separation by distillation of the EGC from the extraction solvent. In the case of catalyst-laden crude products, there is additionally the risk during the extraction that residues of the solvent used for the extraction pass into the reaction circulation during the return of catalyst where they can lead to by-products and decomposition reactions.
It has also already been attempted to purify EGC by recrystallization from a solvent, for example from toluene (U.S. Pat. No. 2,994,705). The disadvantage of such a process is that, to achieve a sufficient purity, repeated recrystallization is necessary. A further disadvantage, which becomes conspicuous in a corresponding manner and as in the extraction, is the necessity of the separation of any catalyst present from the recrystallization solvent.
In a still further method for the purification of crude EGC, this is crystallized in a continuous manner in counter-current from ethylene glycol as solvent (DE-AS (German Published Specification) 12 72 932). In this case, a preheated EGC solution is cooled in the upper part of a crystallization column until the formation of EGC crystals, while the lower part of the column is kept at a temperature of 40.degree. to 50.degree. C. The heating and cooling are coordinated with each other in such a way that a downwards directed crystal flow results; the purified EGC is taken off in the molten state at the column bottom, and the exhausted mother liquor is taken off at the upper part of the column. The crystalline state of the EGC in this purification process is only a briefly occurring intermediate step. By this process, purities of 99.67% EGC (according to gas-chromatographic analysis) are achieved, if the procedure starts from a reaction mixture which arises in the preparation of EGC from ethylene glycol and phosgene. In addition to EGC and glycol, ethylene chlorohydrin and hydrochloric acid are further present in these reaction mixtures. These contents in the crude product are not considered in DE-AS (German Published Specification) '932 itself; however, these can be inferred from Rev. Chim. (Bucharest) 17 (1966), 482-485 where the process described in DE-AS (German Published Specification) '932 is worked out in more detail. If, according to this, mixtures of 50 to 60% EGC, 30 to 40% ethylene glycol, 1% hydrochloric acid, 4 to 5% ethylene chlorohydrin and 5% water are used, a purified EGC arises having the following values: 96.4-98.3% EGC, 0.65-1.41% ethylene glycol, 0.14-0.28% ethylene chlorohydrin and 0.19-0.39% water. However, a purified EGC is thus present whose very high residues of ethylene glycol, ethylene chlorohydrin and, in particular, water can lead in the further use of the EGC to its decomposition and to side reactions.
In a counter-current crystallization from ethylene glycol as that described, in the case of catalyst-laden crude EGC, the catalyst after the crystallization would be present together with the minor components dissolved in the ethylene glycol. If it is desired to return the catalyst to the preparation reaction, a step to separate the catalyst from the ethylene glycol would have to be carried out, since the ethylene glycol, in the presence of the catalysts used and the operating conditions to be maintained in this case, would lead to side reactions of the ethylene oxide. This represents an extremely disadvantageous burden of such a crystallization.
It was therefore desirable to have available a purification process for contaminated EGC which is energy-saving and material-saving, which leads neither to decomposition of the EGC itself nor of the catalyst and, moreover, is to be operated without additives, such as solvents for extraction or crystallization.